1. Field of the Invention
The present invention relates to a high frequency device having a first filter, a frequency converter, and a second filter, and in particular to a high frequency device which requires adjustment of filter characteristics.
2. Description of the Related Art
High frequency devices, such as those used in a receiver (super-heterodyne system), generally have filters for both the RF (radio frequency band) stage and IF (intermediate frequency) stage. Some devices of this type require adjustment of filtering characteristics, or the like, of the filter in the RF stage after assemblage of the circuit. In the following, while referring to FIG. 1, a structure of a conventional device disclosed in Japanese Patent Laid-open Application No. Hei 11-88122 and a method for adjusting characteristics of the filter in the RF stage will be described.
A high frequency device 80 shown in FIG. 1 has an RF stage section 14 which comprises high frequency filters 16, 20 and an amplifier 18. A signal passed through the high frequency filter 20 is supplied to a mixer 26 for frequency conversion whereby a signal having a frequency in an RF band is converted to have a frequency in an IF band. An IF stage section comprises an intermediate frequency filter 34 and an amplifier 42.
Here, the frequency characteristics (filtering characteristics) of the high frequency filters 16, 20 are adjusted with reference to a signal output from an output terminal 44 at the last stage in the IF-stage section. This adjustment, however, is difficult to achieve with high precision while the intermediate frequency filter 34 is engaged with a normal operation, because the intermediate frequency filter 34 affects the signal from the output terminal 44.
In order to address this problem, the high frequency device 80 is normally provided with a circuit 82 for suppressing the influence of the intermediate frequency filter 34. The circuit 82 comprises a serially connected electric switch 84 and resistance R3 and connects both sides of the intermediate frequency filter 34. When the electric switch 84 remains in an ON state, both sides of the intermediate frequency filter 34 are shorted whereby frequency characteristics relating to a portion including the intermediate frequency filter 34 and the circuit 82 are flattened.
In other words, provision of the circuit 82 enables more precise adjustment of the filtering characteristics of the high frequency filters 16, 20, as compared to a structure without the circuit 82. Note that the electric switch 84 remains in an OFF state while the high frequency device 80 is in normal operation. In other words, the circuit 82 is additionally provided so as to operate only during adjustment of the filtering characteristics.
In the above-described conventional structure, however, the amplitude of a signal is attenuated by the circuit 82 having the electric switch 84, and the frequency characteristics relating to the portion including the intermediate frequency filter 34 and the circuit 82 are not thoroughly flattened. This leads to a problem that the precision of the frequency adjustment cannot be significantly improved. Further, because parasitic capacitance relating to the circuit 82 varies depending on whether the electric switch is turned on or off, frequency characteristics of the circuit 82 are different between when the high frequency device 80 is in normal operation and in adjustment operation. Still further, attenuation of a signal amplitude makes proper adjustment difficult, and may delay completion of adjustment.